Polymeric films are used in a wide variety of applications. Multilayer polymeric optical films are widely used for various purposes, including as mirrors and polarizers. These films often have extremely high reflectivity, while being lightweight and resistant to breakage. Examples of a wide variety of multilayer films are included in the commonly assigned U.S. Pat. No. 5,882,774, entitled “Optical Film”. Exemplary applications include compact electronic displays, including liquid crystal displays (LCDs) placed in mobile telephones, personal data assistants, computers, televisions and other devices.
One type of polymer that is useful in creating polarizers or mirror films is a polyester. One example of a polyester-based polarizer includes a stack of polyester layers of differing composition. Such stacks are also commonly referred to as a multilayer reflective film. The multilayer reflective films may also include one or more additional layers which, for example, cover at least one surface of the stack of layers to prevent damage to the stack during or after processing.
A polyester is prepared by reactions of one or more different carboxylate monomers (e.g., compounds with two or more carboxylic acid or ester functional groups) with one or more different glycol monomers (e.g., compounds with two or more hydroxyl functional groups). The properties of a polyester polymer or film vary with the particular choice of kind and amount of monomer molecules.
U.S. Pat. No. 4,904,755 describes low birefringent polyesters that are useful in optical devices. The polyesters include units having opposite optical anisotropies. By varying the ratio of the units in the copolymer, the birefringence of an article made from the copolymer can be controlled as desired. The polyester is a substantially optically anisotropic polyester having diol repeating units derived from 9,9-bis(4-hydroxyphenyl)fluorene and at least about 40% by weight of the diacid repeating units derived from aliphatic or cycloaliphatic dicarboxylic acids, the remainder of the diacid repeating units being derived from aromatic dicarboxylic acids.
Diol repeat units derived from 9,9-bis(4-hydroxyphenyl)fluorene is one example of a class of structures commonly known as Cardo structures.